1. Field of the Invention
The present invention relates to a nonreciprocal circuit device such as an isolator or a circulator used in a high-frequency band such as the microwave band, and to a communication device using the same.
2. Description of the Related Art
Generally, the nonreciprocal circuit device such as a concentrated-constant type isolator or circulator is used in communication devices and the like, taking advantage of its characteristics that it exhibits a very low attenuation in the transmission direction of a signal, while it exhibits a very high attenuation in the opposite direction.
FIG. 9 is an exploded perspective view showing a conventional isolator, and FIG. 10 shows the inner structure thereof. In this isolator, a case 7 made of resin is disposed on a U-shaped lower yoke 8 which has right and left side-walls 8b and a bottom wall 8a, a magnetic assembly 5 comprising center conductors 51, 52, and 53, and a magnetic body 55, capacitors for matching C1, C2, and C3, and a terminating resistor R are provided in the terminal case 7, a permanent magnet 3 is disposed thereon, and a box-shaped upper yoke 2 is mounted on a lower case 8 so as to cover the upper portion in its entirety. The upper yoke 2 and the lower yoke 8 are connected together at two opposing side-walls of the upper yoke 2 and the two side-walls 8b of the lower yoke 8, and form a substantially square-tubular shaped yoke which has a top wall, a bottom wall, and two side-walls continuous with the top and bottom walls. These upper and lower yokes 2 and 8, and the permanent magnet constitutes a magnetic closed circuit.
All center conductors 51 through 53 are arranged so that the common ground portion thereof abuts against the bottom surface of the magnetic body 55, and that these center conductors intersect one another at an angle of substantially 120 degrees on the top surface of the magnetic body 55, passing through the side surfaces of the magnetic body 55. The ports P1 and P2 of the respective center conductors 51 and 52 are connected to input/output terminals 71 and 72 and capacitors C1 and C2 each formed in the terminal case 7. The port P3 of the center conductor 53 is connected to the capacitor C3 and the terminating resistor R, and one-side ends of all capacitor C1 through C3 and the terminating resistor R are connected to a ground terminal 73. The common ground portion of all center conductors 51 through 53 is connected to the bottom wall 8a of the lower yoke 8.
One center conductor 53 is disposed orthogonally to the opposing side-walls 8b of the upper and lower yokes 2 and 8, and the other center conductors 51 and 52 are disposed so as to intersect the center conductor 53 at an angle of substantially 120 degrees.
In such a nonreciprocal circuit device, as shown in FIG. 11, the current i flowing through each of the center conductors (in FIG. 11, only the nonreciprocal circuit device 53 is shown) is split into the upper yoke and the lower yoke from the ground portion. At this time, if the center conductors 51 through 53 having the same conductor width and inter-conductor spacing are used, the current flowing from the center conductor 53 which is disposed orthogonally to the side-walls 8b of the lower yoke 8, to the upper yoke 2 will be larger than the current flowing from the two other center conductors 51 and 52 to the upper yoke 2, while the current flowing from the center conductor 53 to the lower yoke 8 will be smaller than the current flowing from the two other center conductors 51 and 52 to the lower yoke 8. This is because the center conductor 53 is disposed closer to the side-walls 8b of the yoke than the two other center conductors 51 and 52, and because the current path thereof on the upper yoke 2 becomes shorter. As a result, the magnetic flux passing through the magnetic body 55 due to the current i3 of the center conductor 53 orthogonal to the side-walls 8b of the yoke, becomes smaller than the magnetic flux passing through the magnetic body 55 due to each of the current i1 and i2 of the respective other center conductors 51 and 52. Thereby, the coupling of the center conductor 53 orthogonal to the side walls 8b of the yoke, with the other center conductors 51 and 52 becomes weak, and thereby the impedance at the port of the center conductor 53 (hereinafter, referred to as a xe2x80x9cport impedancexe2x80x9d) becomes higher than the impedance of the two other center conductors 51 and 52.
Typically, the impedance of an input/output portion in the circuit where these nonreciprocal circuit devices are used has a predetermine value (usually 50 xcexa9), and the impedance at each port in the nonreciprocal circuit device is also set to be a predetermined equal value.
However, in the conventional center conductor, as described above, since the center conductor disposed orthogonally to the side-walls of the yoke has a higher port impedance than two other center conductors, a problem occurs that the reflection characteristics of the port of this center conductor deteriorates.
Japanese Unexamined Patent Application Publication No. 7-307603 discloses a nonreciprocal circuit device wherein the conductor widths and the inter-conductor spaces of three center conductors are set for each port, but the dispositional relation between these center conductors and the side-walls of a yoke is not treated for this nonreciprocal circuit device. The present inventor has investigated the dispositional relation between the center conductors and the side-walls of the yoke in order to solve the above-described problem.
It is, therefore, an object of the present invention to provide a nonreciprocal circuit device which has an improved reflection characteristics of the port of the center conductor which is disposed orthogonally to the side-walls of a yoke, and to provide a communication device using the same.
In order to achieve the above-described object, the present invention provides a nonreciprocal circuit device comprising a yoke having a top wall, a bottom wall, and two opposing side-walls connected with said top wall and said bottom wall, said yoke accommodating a permanent magnet and a magnetic body, in which the magnetic body includes three center conductors arranged to intersect one another at a predetermined angle, provided in an electrically insulated state from one another, and one of which is disposed substantially orthogonal to the side-walls of the yoke.
In this nonreciprocal circuit device, the conductor width of the center conductor which is disposed substantially orthogonal to the side-walls of the yoke is set to be wider than the conductor width of each of the two other center conductors. Furthermore, when each of the center conductors is constituted of a plurality of conductors, the inter-conductor spacing of the center conductor which is disposed substantially orthogonal to the side-walls of the yoke, is set to be wider than the inter-conductor spacing of each of the two other center conductors.
In accordance with this construction, the port impedance of the center conductor disposed substantially orthogonal to the side-walls of the yoke decreases, and thereby the reflection characteristics of this center conductor can be improved. Specifically, in the present invention, in order to bring the port impedance of the center conductor which is disposed substantially orthogonal to the side-walls of the yoke close to the impedance of the two other center conductors, the conductor width or the inter-conductor spacing of the center conductor which is disposed substantially orthogonal to the side-walls of the yoke, is set to be wider than the conductor width or the inter-conductor spacing of each of the two other center conductors. This allows each port to achieve an appropriate impedance matching, which results in improved reflection characteristics at the port of each of the center conductors.
Preferably, each of the center conductors is constituted of two conductors. This allows insertion loss to be reduced by a simple structure.
Also, it is preferable that a terminating resistor be connected to any one of the center conductors to form an isolator. In this case, since the port of the center conductor which is disposed substantially orthogonal to the side-walls of the yoke has an impedance more prone to deviate than the impedances of the port of the other center conductors, the port of this center conductor is suitable for an isolation port which can be terminated by a resistor having an arbitrary resistance value. It is, therefore, preferable to connect a terminating port to this port.
Moreover, a communication device in accordance with the present invention is implemented by comprising a nonreciprocal circuit device having the above-described features. A communication device having superior characteristics can thereby be achieved.
The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.